When shipping or storing many types of materials, particularly electronic components, it is desirable to know whether those components have been exposed to a particular dose or amount of humidity, since exposure to excessive humidity can cause damage to those components. For example, electronic components can be damaged by exposure to even low levels of humidity and even where the exposure is only for short periods of time. Mounting to a printed circuit board of even a single electronic component that has been exposed to excess humidity can damage the entire board. Electronic components can also be contaminated and/or damaged by exposure to ambient gases, dust and other particulate matter that may be carried to electronic components by atmospheric water vapor particles.
Due to this moisture sensitivity, various humidity indicators have been developed to help users quantitatively assess the integrated cumulative dose of humidity to which a part or article has been exposed, e.g., outside or inside a shipping or storage container.
There are three general classes of humidity indicators. One class of humidity indicators is constituted by reversible humidity indicators (RHIs) that change their color upon exposure to specific humidity levels. Such RHIs typically utilize cobalt chloride as the humidity indicator material. Cobalt chloride changes color when exposed to predetermined levels of humidity and returns to its original color when the humidity level drops below that predetermined level. RHIs can be used to indicate the current condition of a desiccant and/or the current humidity level within a storage container.
Irreversible maximum humidity indicators (MHIs) are a second type of humidity indicator. MHIs indicate if a predetermined level of humidity was reached even for a short period, and can be used, for example, inside a storage container. This indication will not change even if the level of humidity drops below the predetermined level when checked at a later time. Large changes in humidity levels may be experienced by storage containers when they are used in relatively warm climates characterized by dramatic rises and falls in moisture level depending on the temperature of the surrounding air, particularly when an article is packed in a cold environment and moved through warmer climates. This phenomenon may be encountered by materials with different humidity sorption or storage characteristics in the same container. Under such highly variable humidity conditions, a RHI might fail to indicate the temporary presence of high humidity within a storage container even though such high humidity may have been sufficient to cause damage to the components present in the storage container.
The third type of humidity indicator is an irreversible humidity dose indicator (HDI). An HDI displays a measure of the time integral of the level of humidity, H, to which an item was exposed, in accordance with the equation:D=∫H*dt  (1)in which D is the humidity dose, and t is time. The humidity dose to which a part, a drug or food has been exposed is typically what causes them to deteriorate, whether they are in a package or unpackaged. Therefore, the humidity dose is a most accurate measure of the cause of the deterioration of materials. Even if the part is stored in a hermetically sealed package, the humidity changes inside the package due to changes in the temperature and differences in the sorption energy of water on different surfaces within the container. An HDI provides a quantitative cumulative measure of the humidity exposure. Use of an HDI gives the user a better measure of the cumulative deterioration of a stored item. This is a very important measure when storing items such as electronic parts, drugs, artillery shells, missiles, etc, particularly when the structure or packaging includes metal constituents. Such metal structure or packaging is corroded faster at greater humidity, and the total corrosion relates to the integral dose of humidity to which the metal was exposed. The indication of the HDI continues to increase even if the level of humidity drops, since it represents an integrated value.
One of the first MHI devices was disclosed in U.S. Pat. No. 2,214,354, which describes the use of a calcium chloride material that is mixed with a water soluble dye and deposited on a porous surface material, such as a sheet of absorbent paper. Upon exposure of the absorbent sheet to a predetermined humidity level, the calcium chloride material liquefies and releases the dye in liquid form. The dye then is carried by capillary action onto the porous surface of the absorbent paper, where it produces a permanent and irreversible dye mark. This patent also describes various deliquescent agents that may be employed to indicate different humidity levels.
To maintain a consistently low humidity level, shipping containers and long term storage containers usually contain desiccant materials. These desiccant materials dehydrate the storage area and are intended to maintain the humidity level within that storage area at a predetermined low level. These containers are periodically opened to recharge or replace the desiccant materials placed within the container and/or to check the level of humidity in the storage container. After replacing the desiccant material, the container is again sealed. In order to determine whether the humidity level in these storage containers has ever reached certain critical levels, MHIs are also frequently placed within the containers together with the desiccant materials. These MHIs can be viewed at the same time that the desiccant material is being checked to determine whether a harmful humidity level has ever been reached in the shipping container.
Various RHIs, utilizing different deliquescent salts, are disclosed in a series of patents that issued in the 1940's and 1950's including U.S. Pat. Nos. 2,460,065; 2,460,066; 2,460,067; 2,460,068; 2,460,069; 2,460,070; 2,460,071; 2,460,072; 2,460,073; 2,460,074; 2,526,938; 2,580,737; and 2,627,505. Some RHI cards are capable of showing different levels of humidity on the same card by use of a series of different deliquescent agents that change color at varying humidity levels, as disclosed in U.S. Pat. No. 2,249,867.
Humidity indicator sheets and cards that contain deliquescent salts and dyes have commonly been used to detect the relative humidity level present within storage containers. See for example, U.S. Pat. Nos. 2,249,867; 4,034,609; 4,150,570; and 4,854,160. Button-type humidity indicators or “plug” humidity indicators are also sometimes used with packaging material and are disclosed, for example, in U.S. Pat. Nos. 2,716,338; 3,084,658; and 4,050,307. Another device for monitoring humidity levels, particularly in poured cement, is disclosed in U.S. Pat. No. 3,680,364.
A multiple layer, reversible humidity sensing device containing a reflective layer, which is useful in viewing changes in color of a humidity indicator card, is disclosed by U.S. Pat. No. 4,034,609.
A reversible humidity indicator card contained within transparent, flexible sheet materials with an impermeable front layer is disclosed in U.S. Pat. No. 5,224,373. This humidity indicator card is specifically designed for utilization with an electronic meter that can “see” the color changes through a “window” in a barrier bag.
A delayed action MHI card is disclosed in U.S. Pat. No. 4,793,180.
All MHI cards hitherto known are based on combinations of deliquescent salts and water-soluble dyes. In order to prepare humidity indicator cards that react at various humidity levels, different combinations of deliquescent salts and dyes must be chosen. Only cards that show the same change in color at each chosen humidity level are acceptable to users of these cards. Otherwise, if there are varying color changes, it may be difficult to determine whether a specific humidity level has been reached. W. B. Abel: Chemical Maximum Humidity Indicator Update Report, BDX-613-1989 and U.S. Pat. No. 3,898,172 teach that only certain combinations of salts and dyes are useful for this purpose, since the dye is quite soluble in the saturated salt solution that is formed upon deliquescence. In addition, the solubility and color of the dye must be independent of pH changes that may be attributed to the deliquescence of the salt. To ensure a proper shelf-life of the indicator card, the dye also must not react with the salt in any way (e.g., by redox reaction, or acid-base reaction). It is quite difficult to use either the same dye or different dyes with the same color and different salts over the entire humidity spectrum.
Mixing the individual salts with the dye is an additional required step for the production of the previously known MHIs. If the chosen salt and dye have different particle sizes, inhomogeneous distribution of the dye in the salt may occur and lead to inhomogeneous color and appearance on the indicating spot of the humidity indicator. This problem can be overcome by milling the salt and dye together, but this is not possible for all salt-dye combinations, especially if the salt already holds water of crystallization.
Few HDIs have been disclosed in the patent or the open literature. One of the most important HDIs is disclosed in U.S. Pat. No. 4,098,120 (assigned to Minnesota Mining and Manufacturing Company). This patent describes a cumulative humidity indicating device that visibly indicates the total exposure to humidity. The device of this patent comprises, in combination, a deliquescent compound, a liquid absorbent wick, and an indicating means. The liquefaction of the deliquescent compound when it absorbs water at a particular humidity level allows it to wick. The progress of the liquid front through the wick indicates the integrated amount of humidity that has been absorbed, above the absorption threshold of the deliquescent material.